An air pollution control system for treating flue gas discharged from a boiler installed in a thermal power plant, or the like, has been conventionally known. Such an air pollution control system includes: a denitrator for removing nitrogen oxides from the flue gas from the boiler; an air heater for recovering heat of the flue gas passed through the denitrator; a precipitator for removing ash dust in the flue gas after the heat recovery; and a desulfurizer for removing sulfur oxides in the dust-removed flue gas. A typical desulfurizer used is a wet-type desulfurizer that removes sulfur oxides in flue gas by causing gas-liquid contact between limestone absorbent, for example, and the flue gas.
Effluent discharged from the wet-type desulfurizer (hereinafter it is referred to as “absorber slurry.”) contains a large amount of various kinds of toxic substances including ions, such as chlorine ions or ammonium ions, and mercury. Thus, there is a need to remove these toxic substances from the desulfurized effluent before such desulfurized effluent is discharged to the outside of the system. However, treatment for removing such many kinds of toxic substances contained in the desulfurized effluent is a complicated process and its treatment cost is therefore high. In view of this, a method for treating desulfurized effluent within a system without discharging the desulfurized effluent to the outside of the system has been proposed to save the treatment cost of the desulfurized effluent. For example, Patent Literatures 1 and 2 each disclose an air pollution control apparatus in which a facility for spraying and gasifying desulfurized effluent is separately installed so as to be branched from a main line flue gas duct to which a denitrator, an air heater, a precipitator, and a desulfurizer are connected, part of flue gas is introduced into this facility from the main line flue gas duct, desulfurized effluent is sprayed and evaporated in the flue gas in the facility to precipitate toxic substances, and then the gas is returned to the main line flue gas duct (Patent Literatures 1 and 2).